1. Field of the Invention
The present invention relates to a discharge head, a method of manufacturing a discharge head, and a liquid discharge apparatus, and more particularly to a structure and manufacturing technology for a discharge head that is suitable for discharging liquid of high viscosity and achieving high-speed discharge.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. By driving recording elements, such as nozzles, provided in a recording head in accordance with data, an inkjet printer is able to form data onto a recording medium, such as recording paper, by means of ink discharged from the nozzles.
In an inkjet printer, a desired image is formed on a recording medium by causing a recording head having a plurality of nozzles and a recording medium to move relative to each other, while causing ink droplets to be discharged from the nozzles.
In an inkjet recording apparatus, actuators made of lead zirconate titanate (PZT) piezoelectric elements, or the like, are provided as ink discharge pressure application devices for discharging ink droplets from the print head. By deforming the liquid chambers (pressure chambers) containing the ink to be discharged, by driving the actuators, a pressure is applied to the ink inside the liquid chambers.
The ink has a property whereby the ink solvent evaporates and the viscosity of the ink increases when it is in contact with air. Therefore, in an inkjet recording apparatus based on the system described above, if a nozzle does not perform ink discharge for a prescribed period of time, the ink inside the nozzle may increase in viscosity, it may become impossible to discharge droplets at the prescribed discharge frequency, and discharge abnormalities may arise, such as discharge failures in which no ink droplet is discharged, even if a prescribed discharge pressure is applied.
In general, in order to discharge ink of high viscosity (high-viscosity liquid), a higher pressure (discharge pressure) should be applied when discharging the ink, in comparison to the prescribed discharge pressure. However, there are limitations on the pressure which can be applied to the ink, due to the characteristics of the actuators and the shape of the liquid chambers. Furthermore, since the ink flow velocity is slower in high-viscosity liquid that in ink having a low viscosity, there is a tendency for the time required for refilling (the replenishment time) to increase, and hence it is difficult to increase the droplet ejection (discharge) frequency.
Various factors, such as the structure and shape of the actuators and the structure and shape of the liquid chambers, can be devised in order to achieve satisfactory discharge of high-viscosity liquid of this kind.
On the other hand, demands for increased printing speed are met by raising the droplet ejection frequency of the print head by modifying the drive waveform of the actuators, as well as modifying the structure and control of the print head, and the composition of the conveyance device and the conveyance control used for same.
In the inkjet head described in Japanese Patent Application Publication No. 2001-58401, a portion of the wall of a liquid chamber is formed by a diaphragm, and a piezoelectric element including a lower electrode, a piezoelectric film and an upper electrode is provided on the diaphragm. The shape and size of the liquid chamber, the thickness of the diaphragm, and the thickness of the piezoelectric film are all optimized, for instance, the piezoelectric film is formed to a greater thickness than the diaphragm, in such a manner that freedom of design can be increased without causing fluctuations in the spraying characteristics.
Furthermore, in the inkjet head described in Japanese Patent Application Publication No. 10-217466, a diaphragm is provided on the upper surface of the liquid chamber, and a lower electrode, a piezoelectric element and an upper electrode are provided, in this order from the bottom upward, on top of the diaphragm, in such a manner that the piezoelectric element is sandwiched between the upper electrode and the lower electrode. Since the width of the shorter edge of the lower electrode is smaller than the width of the shorter edge of the diaphragm, and the width of the longer edge of the lower electrode is greater than the width of the longer edge of the diaphragm, the displacement of the diaphragm is increased and it becomes possible to increase the volume of the discharged ink droplet.
However, the actuators, such as piezo-type piezoelectric elements, may be restricted by the design of the print head, namely, the size, shape and structure of the actuators are determined by the size of the print head, the number of liquid chambers provided in the print head (in other words, the nozzle density), and the like. Furthermore, if a special shape or structure is adopted for the liquid chambers, then the manufacturing steps become complicated and increase in number, and this may lead to increased manufacturing costs and reduced production yield.
In the inkjet head described in Japanese Patent Application Publication No. 2001-58401, the dimensional relationships in the region forming the inkjet head are optimized in such a manner that it is possible to achieve a compact, high-density head that is difficult to manufacture conventionally, as well as improving performance characteristics and achieving greater stability in the manufacturing process. However, there is no disclosure regarding a satisfactory method for controlling an inkjet head having a structure of this kind.
Furthermore, in the inkjet head described in Japanese Patent Application Publication No. 10-217466, the displacement of the diaphragm, and hence the ink discharge volume, is increased by optimizing the sizes of the diaphragms and the piezoelectric elements. However, there is no disclosure regarding the control of the piezoelectric elements, and there is no disclosure regarding issues relating to discharge of high-viscosity liquid or high-speed droplet ejection.